1. Field of the Invention
The invention relates to polyurethane powder coating compositions based on epoxy-terminated polyaddition compounds containing uretdione groups, which cure at low baking temperatures, to a process for preparing such compositions, and to the use of the compositions for producing plastics, including polyurethane powder coatings, which crosslink at low baking temperatures to give high-gloss or matt, light-stable and weather-stable coating films.
2. Description of the Related Art
Externally or internally blocked polyisocyanates which are solid at room temperature are useful as crosslinkers for thermally crosslinkable polyurethane (PU) powder coating compositions.
Thus, for example, U.S. Pat. No. 4,246,380 describes PU powder coatings having outstanding weathering stability and thermal stability. The crosslinkers whose preparation is described in U.S. Pat. No. 4,302,351 are composed of isophorone diisocyanate containing isocyanurate groups blocked with ε-caprolactam. Also known are polyisocyanates containing urethane, biuret or urea groups, the isocyanate groups of these polyisocyanates being likewise blocked.
The drawback of these externally blocked systems lies in the elimination of the blocking agent during the thermal crosslinking reaction. Since the blocking agent may thus be emitted to the environment, it is necessary on environmental and workplace safety grounds to take special precautions in order to clean the outgoing air and/or to recover the blocking agent. Moreover, the reactivity of the crosslinkers is low. Curing temperatures above 170° C. are required.
U.S. Pat. No. 4,463,154 and U.S. Pat. No. 4,483,789 describe processes for preparing polyaddition compounds which contain uretdione groups and whose terminal isocyanate groups are blocked irreversibly with monoalcohols or monoamines. A particular drawback are the chain-terminating constituents of the crosslinkers, which lead to low network densities in the PU powder coatings and hence to moderate solvent resistances.
Uretdione powder coating crosslinkers prepared by reacting polyisocyanates containing uretdione groups with diols and ester-containing and/or carbonate-functional chain extenders, or using dimer diols, are described in U.S. Pat. No. 5,621,064 and in U.S. Pat. No. 5,596,066, respectively.
Hydroxyl-terminated polyaddition compounds containing uretdione groups are subject matter of U.S. Pat. No. 6,613,861. On the basis of their functionality of two they exhibit improved resistance to solvents.
PU powder coatings based on epoxy-terminated polyaddition compounds containing uretdione groups, as are described in EP 1 063 269, possess an even substantially higher chemical resistance.
A feature common to the powder coating compositions based on these polyisocyanates containing uretdione groups is the absence of any emission of volatile compounds during the curing reaction. However the at least 180° C. baking temperatures are high.
The use of amidines as catalysts in PU powder coating compositions is described in U.S. Pat. No. 5,847,044. Although these catalysts do lead to a reduction in the curing temperature, they exhibit considerable yellowing, which is generally undesirable in the coatings sector. The cause of this yellowing is thought to be the reactive nitrogen atoms in the amidines. These are able to react with atmospheric oxygen to form N-oxides, which are responsible for the discoloration.
U.S. Pat. No. 5,847,044 also mentions other catalysts which have been used to date for this purpose, but without indicating any special effect on the cure temperature. They include the organometallic catalysts known from polyurethane chemistry, such as dibutyltin dilaurate (DBTL), or else tertiary amines, such as 1,4-diazabicyclo[2.2.2]octane (DABCO), for example.
WO 00/34355 claims catalysts based on metal acetyl acetonates: zinc acetyl acetonate, for example. Such catalysts are actually capable of lowering the curing temperature of polyurethane powder coating compositions containing uretdione groups, but their reaction products are principally allophanates (M. Gedan-Smolka, F. Lehmann, D. Lehmann “New catalysts for the low temperature curing of uretdione powder coatings” International Waterborne, High Solids and Powder Coatings Symposium, New Orleans, Feb. 21-23, 2001). Allophanates are the reaction products of one mole of alcohol and two moles of isocyanate, whereas in the conventional urethane chemistry one mole of alcohol reacts with one mole of isocyanate. As a result of the unwanted formation of allophanate, therefore, isocyanate groups valuable both technically and economically are destroyed.
DE 103 20 267, U.S. 2003/0153713, and DE 103 20 266 describe metal hydroxides, metal alkoxides, quaternary ammonium salts with hydroxides, fluorides or carboxylates, which accelerate the unblocking of uretdione groups so greatly that when using powder coating curing agents which contain uretdione groups it is possible to achieve considerable reduction in the cure temperature of powder coating compositions.
Since the activity of these catalysts decreases markedly in the presence of acids, the coatings produced from highly accelerated powder coating compositions of this kind include an acid scavenger. Acid groups are normally present in significant amounts in the polyester polyols employed. Possible examples of acid scavengers used include 2-oxazolines, which are very expensive. It is advantageous, however, to use epoxide compounds. Liquid epoxide compounds, such as diglycidyl ethers based on bisphenol A, for instance, have the drawback that they can only be used in the form of a cost-intensive solid masterbatch. Accordingly particular preference is given to solid epoxide compounds, such as triglycidyl isocyanurate (TGIC), for instance. The powder coating compositions formulated with TGIC, though, are subject to labeling requirements owing to the toxic potential of TGIC. In conventional TGIC-containing powder coating materials the crosslinking temperature is normally above 180° C.